The control of the thicknesses and composition of the different epitaxial layers which participate in a complex semiconducting structure is achieved by acting on the molecular beams by means of appropriate screens or valves. In particular, for the controlled production of molecular beams of arsenic (As.sub.4) from the thermal sublimation of solid arsenic, effusion cells in which the opening/closing screen has been replaced by a rapid-actuation valve maintained at high temperature, to prevent condensation of the As.sub.4 molecules therein, have been designed in the Centro Nacional de Microelectronica [National Microelectronics Centro] (C.S.I.C.) and employed successfully.
The crucible or vessel in which the arsenic is sublimed is leak-tight, except for the above-mentioned valve which opens it to the epitaxy reactor, and an auxiliary valve which connects it to a vacuum system. The vessel is maintained at a uniform temperature of the order of 300.degree.-350.degree. C. to obtain flows of As.sub.4 with an equivalent beam pressure in the range 10.sup.-6 -10.sup.-5 mbar. For the production of molecular beams of As.sub.2, a nozzle provided with tantalum surfaces at high temperature, which are capable of thermally dissociating As.sub.4 molecules to As.sub.2, was added. At the present time, the company EPI.sup.2 [III-Vs Feature Review, 4(3), 31 (1990)] commercially produces a cell for As.sub.4 and As.sub.2 which utilizes the same principle and arrangement as the one described above.
When this type of cell was used for the production of molecular beams of P.sub.4 or P.sub.2 from the sublimation of solid phosphorus (red variety), great difficulties arose, both with the CNM cell and with the EPI cell. In particular, at the operating temperature of the cell 300.degree. C., necessary for obtaining an equivalent beam pressure of the order of 10.sup.-5 -10.sup.-6 mbar in the steady state (valve open), a gradual rise takes place in the internal pressure in the cell (valve closed), leading to a strong equivalent beam pressure transient, or pressure burst, at the time of opening the valve. This transient makes it difficult to control the epitaxial process and is detrimental to the UHV pumps. U.S. Pat. No. 5,156,815 Streetman et al. discloses an arsenic effusion cell, or sublimation and cracking apparatus in which arsenic is sublimed in a primary furnace or sublimator 12 at temperatures of the order of 235.degree. C. and is cracked in a perpendicular cracker at temperatures that are preferably in the range of 750.degree. to 1050.degree. C. The lower end of cracker 14 is described by Streetman et al. as providing a heat sink zone between the cracker and the sublimator (column 4, lines 29-55, column 6, lines 11-24 and column 10, lines 17-31). This heat sink zone, is obtained by restricting the extent of heater filament 78 to an upper portion of cracker 14 and by circulating a, probably gaseous, coolant through a cooling jacket 70 surrounding a lower end of sublimator 12. This exposed, lower heat sink end of cracker tube 18 serves the purpose of minimizing undesired heat transfer between the cracking and sublimator sections.
The heat sink zone is described by Streetman as having a substantially lower operation temperature than the temperature of the primary cracking zone. The lower temperature of the heat sink zone is however substantially above that of the sublimator 12 because of a much higher cracking temperature and because of heat transmission down the cracking tube 18. Streetman et al.'s heat sink zone serves to prevent back transfer of heat from the cracker to the sublimator leading to overpressure due to enhanced sublimation of arsenic. While it may have been suggested to use arsenic effusion cells to generate phosphorus beams, Streetman et al.'s sublimation and cracking apparatus is not designed as a phosphorus effusion cell and provides no means to solve the problems that arise in subliming and cracking phosphorus, specifically the problem of an initial pressure surge and providing a readily controllable, reproducible phosphorus beam.
A further crucial drawback to employing the Streetman et al. teaching for propagating a phosphorus beam for use in MBE is the lack of efficient flux control valving means. Streetman et al. constrained by a high-temperature environment provides only a crude shutter 112 pivoting on a hinge 114, which is neither vacuum-tight nor rapid-acting.
A problem with prior proposals is that they are either unaware of, or have underestimated, the quantity of white phosphorus which is formed by condensation as a result of evaporation of red phosphorus and the pressure impact it creates The P.sub.4 pressure of white phosphorus at 300.degree. C., a suitable red phosphorus evaporation temperature, is close to 1 bar. This is an enormous pressure compared with desired beam equivalent pressures of about four-millionths of a bar.
The high internal white phosphorus pressures of known cell effusion systems, such as those used for arsenic mean that each time the valve between the sublimation furnace and the cracker is opened, a large burst of pressure is produced. The MBE growth chamber into which this burst of pressure is discharged cannot sustain such high pressure and may require several hours to recover to a practical working pressure. Also the large amount of phosphorus blown into the growth chamber will inevitably provide a high residual background of phosphorus even in subsequent processes where phosphorus may not be required.
A further drawback of known systems is that a stable state is only reached after a long period with the valve constantly open. This is an unacceptable impediment to most researchers or fabricators using molecular phosphorus beams. Such instabilities makes it difficult or impossible for researchers and fabricators to rely upon a particular beam equivalent pressure for accurate calibration of Group III and other Group V fluxes, or to gain flux reproducibility.